Published on June 24th, 2016 | by Paul Jennings

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The sound of things to come

The sound a car makes is important.  Read any review of a new car and you will find that the sound it makes (or doesn’t make) invariably gets mentioned. We judge the engine speed (especially before tachometers became standard) by the pitch of the sound it makes. We can tell when it “doesn’t sound right” and needs to be taken to a garage. For those of a more adventurous disposition, the sound of the exhaust note when the engine is working hard adds excitement to the experience of driving. Which is why it is also part of the way we identify the brand of the car. And it is as important from outside the car too. Despite the fact that we complain about the general level of “noise pollution” caused by road vehicles, we have also learned to judge the danger presented by a nearby car from the way we hear it – is it near or far, is it accelerating or decelerating, what size is it, and so on? This latter appreciation of sound is one of the reasons legislation is on its way to ensure electric vehicles emit a certain level of sound. It is not that electric vehicles are silent. There is still the sound made by the interaction of the tyres and the road and the displacement of air as it moves, but at low speed these are not easily distinguished from other urban sounds and so pedestrians would have to use their eyes more to avoid collisions. We have adapted and use the sound we cannot really avoid – made by the internal combustion engine – as a warning signal.

However, if we stop and think, electric cars do offer the opportunity to tailor the sound a car makes to our needs and the chance to separate out the sound of travel inside and outside the vehicle and to make an overall impact on the levels of traffic noise. We might choose to keep the sound inside the car as low as possible, although we would still use it to ensure the driver gets the sort of feedback they are used to, but what about outside? As previously mentioned, early discussions suggested that legislation would need to ensure cars emit a certain level of sound to warn pedestrians of their presence. This approach shows a lack of understanding of what is going on. The sound cars emit must be aligned with what we have learned is a danger or caution signal. A car emitting the sound of a loud songbird would not fulfil the real goal, although it might confirm to the law. And perhaps the car doesn’t need to emit the sound all the time – modern cars have a wide variety of sensors which monitor what is going on nearby (currently dedicated to parking) which could be used to identify pedestrians and only emit sound at the right time – although sudden sounds might not allow our developed senses to use their current learned response and there is an argument that transient sounds are more annoying than continuous ones. There is even the potential for directional sound to only warn those who might be affected by the movement of the car.

All these questions come in a different form when we think about driverless cars. If the driver is taking no part in driving, do they need any sound at all (other than the digitally reproduced sound of the ticking clock)? If they are using the time to work, then perhaps any internal sound (other than their choice of music?) would be a distraction? Outside, the basic need is still the same – to make pedestrians and other road users aware that the car is there and what it is doing. Do we need to be extra careful when driverless cars are introduced – to highlight their existence and use?

As discussed earlier, the need, in in both driven and driverless vehicles, is that the external sound acts as a warning that there is a (potentially dangerous) vehicle nearby. To do that it must conform to what we are used to and associate with a vehicle moving nearby. In some experiments, it has been shown that the sounds used in science fiction films act in a similar way to those from current cars. This suggests that we can learn new cautionary sounds and even design sounds that are less disruptive to the overall soundscape and learn to adapt to them. But then perhaps we just need to get that sort of sound included in films currently being made so that we can know what to expect?

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Professor Paul Jennings has been involved in research with the automotive industry for over 25 years, and now leads research and development on Intelligent Vehicles, Energy and Electrical Systems, and Experiential Engineering.

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